Pneumatic tire

ABSTRACT

In a pneumatic tire, at least one carcass layer mounted between a pair of bead portions is formed from a carcass cord formed of an organic fiber cord having filament bundles of organic fibers intertwined together, a fineness based on corrected mass of the carcass cord is from 4000 dtex to 8000 dtex, an intermediate elongation of the carcass cord at a sidewall portion under 1.0 cN/dtex load is from 3.3% to 4.2%, and a ratio G/R of an interlayer rubber gauge G between a body portion and a folded back portion to a cord diameter R of the carcass cord, at a contact region where the body portion and the folded back portion are in contact in the carcass layer, is from 0.50 to 0.60.

TECHNICAL FIELD

The present technology relates to a pneumatic tire provided with acarcass layer formed of organic fiber cords and particularly relates toa pneumatic tire that enables both steering stability and high-speeddurability.

BACKGROUND ART

A pneumatic tire typically includes a carcass layer mounted between apair of bead portions. Rayon fiber cords are preferably used as thecarcass cords forming such a carcass layer, particularly inhigh-performance tires. In this respect, from the perspective of makinga pneumatic tire lighter in weight, for example, it has been proposed touse polyethylene terephthalate fiber cords (hereinafter referred to asPET fiber cords) that have high strength and are inexpensive comparedwith rayon fiber cords (for example, see Japan Unexamined PatentPublication No. 2015-189252).

In such a pneumatic tire in which polyester fiber cords are used in acarcass layer, in order to further improve steering stability, it hasbeen attempted to increase the fineness of the carcass cords to achievea higher rigidity in the carcass cords (in particular, a higher rigidityin the sidewall portion). However, since carcass cords having anincreased fineness and a high rigidity generate much heat, a significantincrease in the temperature in the sidewall portion (in particular, theregion where the body portion and the folded back portion of the carcasslayer are in contact) occurs, and there is a risk that the adhesivenessbetween the body portion and the folded back portion of the carcasslayer will decrease when traveling at high speeds, and separation mayoccur. Accordingly, in a pneumatic tire provided with a carcass layerformed of organic fiber cords, there is a demand for measures to achievea high level of both steering stability and high-speed durability.

SUMMARY

The present technology is to provide a pneumatic tire provided with acarcass layer formed of organic fiber cords, which enables both steeringstability and high-speed durability.

A pneumatic tire according to an embodiment of the present technologyincludes a tread portion extending in a tire circumferential directionand having an annular shape, a pair of sidewall portions respectivelydisposed on both sides of the tread portion, and a pair of bead portionseach disposed on an inner side of the pair of sidewall portions in atire radial direction, the pneumatic tire including at least one carcasslayer mounted between the pair of bead portions, the carcass layer beingformed from a carcass cord formed of an organic fiber cord havingfilament bundles of organic fibers intertwined together, a fineness ofthe carcass cord after dip treatment being from 4000 dtex to 8000 dtex,and an intermediate elongation of the carcass cord at the sidewallportion under 1.0 cN/dtex load being from 3.3% to 4.2%, and the carcasslayer being formed from a body portion extending from the tread portionthrough each sidewall portion to a corresponding bead portion and afolded back portion folded back at each bead portion and extendingtoward a corresponding sidewall portion side, and having a contactregion at the sidewall portion where the body portion and the foldedback portion are in contact, and a ratio G/R of an interlayer rubbergauge G between a carcass cord within the body portion and a carcasscord within the folded back portion at the contact region to a corddiameter R of the carcass cord being from 0.50 to 0.60.

According to an embodiment of the present technology, as describedabove, the fineness of the carcass cord forming the carcass layer afterdip treatment is from 4000 dtex to 8000 dtex, and the intermediateelongation of the carcass cord at the sidewall portion under 1.0 cN/dtexload is from 3.3% to 4.2%, and thus the rigidity in the sidewall portionis increased and steering stability can be improved. On the other hand,since the ratio G/R described above is set to be in the range of from0.50 to 0.60, and the interlayer rubber gauge G between a carcass cordwithin the body portion and a carcass cord within the folded backportion at the contact region is ensured to be appropriately large withrespect to the cord diameter R of the carcass cord, a temperatureincrease and shear strain in the body portion and the folded backportion of the carcass layer at the contact region are suppressed andhigh-speed durability can be improved. Due to these measures, thepneumatic tire according to an embodiment of the present technology canachieve a high level of both steering stability and high-speeddurability.

According to an embodiment of the present technology, a twistcoefficient K of the carcass cord after dip treatment represented byFormula (1) below is preferably 2000 or more. By having such a largetwist coefficient K, durability against fatigue due to repeatedcompressive deformation of the turned up portion of the carcass layercaused by the flexing of the bead portion when the tire is rolling canbe ensured.K=T×D ^(1/2)  (1)

(where T is an upper twist count of cord (counts/10 cm), and D is thetotal fineness of cord (dtex))

According to an embodiment of the present technology, an organic fiberforming the carcass cord is preferably a polyethylene terephthalatefiber. In this way, the physical properties of the carcass cord arefurther improved, which is advantageous in achieving a high level ofboth steering stability and high-speed durability.

According to an embodiment of the present technology, the fineness ofthe carcass cord after dip treatment is preferably from 5000 dtex to7000 dtex. Also, an intermediate elongation of the carcass cord at thesidewall portion under 1.0 cN/dtex load is preferably from 3.5% to 4.0%.Further, a twist coefficient K of the carcass cord after dip treatmentis preferably from 2100 dtex to 2500 dtex. By determining each physicalproperty value in this manner, the physical properties of the carcasscord are further improved, which is advantageous for achieving a highlevel of both steering stability and high-speed durability.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a meridian cross-sectional view illustrating a pneumaticradial tire according to an embodiment of the present technology.

FIG. 2 is a schematic view illustrating an interlayer rubber gauge G bycombining a meridian cross-sectional view of a portion of the pneumaticradial tire according to an embodiment of the present technology and across-sectional view taken along line X-X in FIG. 2 .

DETAILED DESCRIPTION

Configurations of embodiments of the present technology will bedescribed in detail below with reference to the accompanying drawings.

As illustrated in FIG. 1 , a pneumatic tire of an embodiment of thepresent technology includes a tread portion 1, a pair of sidewallportions 2 disposed on both sides of the tread portion 1, and a pair ofbead portions 3 disposed in the sidewall portions 2 at an inner side ina tire radial direction. Note that “CL” in FIG. 1 denotes a tireequator. Although not illustrated in FIG. 1 as FIG. 1 is a meridiancross-sectional view, the tread portion 1, the sidewall portions 2, andthe bead portions 3 each extend in a tire circumferential direction toform an annular shape. Thus, a toroidal basic structure of the pneumatictire is configured. Although the description using FIG. 1 is basicallybased on the illustrated meridian cross-sectional shape, all of the tirecomponents each extend in the tire circumferential direction and formthe annular shape.

In the illustrated example, a plurality of main grooves (four maingrooves in the illustrated example) extending in the tirecircumferential direction are formed in the outer surface of the treadportion 1; however, the number of main grooves is not particularlylimited. Further, in addition to the main grooves, various grooves andsipes that include lug grooves extending in a tire width direction canbe formed.

A carcass layer 4 including a plurality of reinforcing cords (carcasscords 4 c) extending in the tire radial direction are mounted betweenthe pair of left and right bead portions 3. A bead core 5 is embeddedwithin each of the bead portions, and a bead filler 6 having atriangular cross-sectional shape is disposed on the outer periphery ofthe bead core 5. The carcass layer 4 is folded back around the bead core5 from an inner side to an outer side in the tire width direction.Accordingly, the bead core 5 and the bead filler 6 are wrapped by a bodyportion 4A (a portion extending from the tread portion 1 through each ofthe sidewall portions 2 to a corresponding one of the bead portions 3)and a folded back portion 4B (a portion folded back around the bead core5 of each bead portion 3 to extend toward a corresponding one of thesidewall portions 2) of the carcass layer 4.

A plurality (in the illustrated example, two layers) of belt layers 7are embedded on an outer circumferential side of the carcass layer 4 inthe tread portion 1. The belt layers 7 each include a plurality ofreinforcing cords (belt cords) inclined with respect to the tirecircumferential direction, and are disposed such that the belt cords ofthe different layers intersect each other. In these belt layers 7, theinclination angle of the belt cords with respect to the tirecircumferential direction is set in a range of, for example, 10° or moreand 40° or less. Steel cords are used as the belt cords, for example.

A belt cover layer 8 is provided on an outer circumferential side of thebelt layer 7 for the purpose of improving high-speed durability andreducing road noise. The belt reinforcing layer 8 includes reinforcingcords (belt reinforcing cords) oriented in the tire circumferentialdirection. In the belt reinforcing layer 8, the angle of the beltreinforcing cords with respect to the tire circumferential direction isset, for example, to from 0° to 5°. Organic fiber cords (for example,PET fiber cords) are used as the belt reinforcing cords. In anembodiment of the present technology, the belt cover layer 8 necessarilyincludes a full cover layer 8 a that covers the entire region of thebelt layers 7, and can be configured to include a pair of edge coverlayers 8 b that locally cover both end portions of the belt layers 7 asnecessary (in the illustrated example, the belt cover layer includesboth the full cover layer 8 a and the edge cover layers 8 b). The beltcover layer 8 is preferably configured such that a strip material madeof at least a single belt reinforcing cord bunched and covered withcoating rubber is wound helically in the tire circumferential direction,and desirably has, in particular, a jointless structure.

The present technology relates to the carcass cord forming the carcasslayer 4 described above, and thus the basic structure of the entire tireis not limited to that described above.

According to an embodiment of the present technology, the carcass cord 4c forming the carcass layer 4 is formed of an organic fiber cord havingfilament bundles of organic fibers intertwined together. The fineness ofthe carcass cord 4 c (organic fiber cord) after dip treatment is from4000 dtex to 8000 dtex, and preferably from 5000 dtex to 7000 dtex.Also, the intermediate elongation of the carcass cord 4 c at thesidewall portion 2 under 1.0 cN/dtex load is from 3.3% to 4.2%, andpreferably from 3.5% to 4.0%. The type of organic fibers constitutingthe carcass cords 4 c is not particularly limited, and for example,polyester fibers, nylon fibers, aramid fibers, or the like can be used,with polyester fibers being particularly suitable. Additionally,examples of the polyester fibers include polyethylene terephthalatefibers (PET fibers), polyethylene naphthalate fibers (PEN fibers),polybutylene terephthalate fibers (PBT), and polybutylene naphthalatefibers (PBN), with PET fibers being particularly suitable. Note that“fineness after dip treatment” is a fineness measured after performingdip treatment on the carcass cord 4 c (organic fiber cord), and is not avalue of the carcass cord 4 c (organic fiber cord) itself, but rather avalue of that also including dip liquid adhered to the carcass cord 4 c(organic fiber cord) after dip treatment. Additionally, “intermediateelongation under 1.0 cN/dtex load” is the elongation ratio (%) of samplecords, which is measured under 1.0 cN/dtex load by conducting a tensiletest in accordance with JIS (Japanese Industrial Standard)-L1017 “testmethods for chemical fiber tire cords” and under the conditions that thegripping interval is 250 mm and the tensile speed is 300±20 mm/minute,for carcass cords 4 c (sample cords) extracted from the sidewall portion2 of the pneumatic tire.

Also, since the carcass layer 4 is formed from the body portion 4A andthe folded back portion 4B as described above, the carcass layer 4 has acontact region A at the sidewall portion 2 where the body portion 4A andthe folded back portion 4B are in contact. The range of the contactregion A is not particularly limited and can be, for example, in a rangeof from 2 mm to 15 mm from the outer edge of the bead filler 6 in thetire radial direction toward the outer side in the tire radialdirection. As illustrated in FIG. 2 , in the contact region A, when thesidewall portion 2 is cut along the tire circumferential direction andan interlayer rubber gauge G is a distance between a carcass cord 4 cincluded in the body portion 4A and a carcass cord 4 c included in thefolded back portion 4B, a ratio G/R of the interlayer rubber gauge G toa cord diameter R of the carcass cord 4 c is from 0.50 to 0.60, andpreferably from 0.52 to 0.58. Note that in a cross section where thesidewall portion 2 is cut along the tire circumferential direction,variation may occur in the carcass cords 4 c, and thus the interlayerrubber gauge G is obtained, as illustrated in FIG. 2 , by drawing anapproximate straight line through the centers of the plurality ofcarcass cords 4 c included in the body portion 4A and an approximatestraight line through the centers of the plurality of carcass cords 4 cincluded in the folded back portion 4B so as to be parallel with eachother, and subtracting the cord diameter R of the carcass cord 4 c fromthe distance L between the approximate straight lines.

In this manner, by using the organic fiber cord having specific physicalproperties in the carcass layer 4 and setting the interlayer rubbergauge G at the contact region with respect to the cord diameter R asdescribed above, the pneumatic tire according to an embodiment of thepresent technology can achieve a high level of both steering stabilityand high-speed durability. In other words, by the carcass layer 4(organic fiber cord) having the physical properties described above, therigidity in the sidewall portion 2 can be increased, and steeringstability can be improved. On the other hand, the interlayer rubbergauge G at the contact region A is ensured to be appropriately largewith respect to the cord diameter R of the carcass cord 4 c, and thustemperature increase and shear strain in the body portion 4A and thefolded back portion 4B of the carcass layer 4 at the contact region A issuppressed and high-speed durability can be improved.

In this situation, when the fineness of the carcass cord 4 c after diptreatment is less than 4000 dtex, the carcass cord 4 c will be toonarrow and its rigidity cannot be sufficiently ensured, and thus theeffect of improving steering stability cannot be obtained. When thefineness of the carcass cord 4 c after dip treatment is greater than8000 dtex, heat generation caused by the carcass cords 4 c cannot besuppressed, and high-speed durability will decrease. When theintermediate elongation described above is less than 3.3%, the rigidityof the sidewall portion 2 becomes excessive, and high-speed durabilitywill decrease. When the intermediate elongation described above isgreater than 4.2%, the rigidity of the sidewall portion 2 cannot besufficiently ensured, and the effect of improving steering stabilitycannot be obtained. When the ratio G/R described above is less than0.50, the interlayer rubber gauge is excessively small, and thus thetemperature increase and shear strain between the body portion 4A andthe folded back portion 4B of the carcass layer 4 cannot be sufficientlysuppressed, and high-speed durability will decrease. When the ratio G/Rdescribed above is greater than 0.60, the rigidity of the sidewallportion 2 becomes excessive, and the high-speed durability willdecrease.

In addition to the physical properties described above, in the carcasscord 4 c according to an embodiment of the present technology, the twistcoefficient K represented by the Formula (1) below is preferably 2000 ormore, and more preferably from 2100 to 2500. Note that the twistcoefficient K is a value of the carcass cord 4 c after dip treatment.Setting the twist coefficient K to be large in this manner isadvantageous in improving high-speed durability. When the twistcoefficient K is less than 2000, repeated compressive deformation of theturned up portion of the carcass layer 4 caused by flexing of the beadportion when the tire is rolling may cause fatigue to occur in thecarcass layer 4, and there is a risk that the effect of improvinghigh-speed durability will not be sufficiently obtained.K=T×D ^(1/2)  (1)

(where T is an upper twist count of cord (counts/10 cm), and D is thetotal fineness of cord (dtex))

Examples

Pneumatic tires according to Conventional Example 1, ComparativeExamples 1 to 5 and Examples 1 to 7 were manufactured. The tires have atire size of 255/35R19 and include a basic structure illustrated in FIG.1 . The tires are different in terms of the fineness of the carcass cordafter dip treatment, the intermediate elongation of the carcass cord atthe sidewall portion under 1.0 cN/dtex load, the ratio G/R of theinterlayer rubber gauge G between a carcass cord within the body portionand a carcass cord within the folded back portion at a contact region toa cord diameter R of the carcass cord, and a twist coefficient K asindicated in Tables 1 and 2.

Note that in all examples, the rubber gauge G was measured in a crosssection in which the sidewall portion was cut in the tirecircumferential direction at a position 5 mm on the outer side in thetire radial direction from the outer edge of the bead filler in theradial direction. Also, in all examples, polyethylene terephthalatefibers (PET fibers) were used as the organic fibers forming the carcasscord.

These test tires were evaluated for steering stability, high-speeddurability, and presence of separation according to the followingevaluation methods. The results are shown in Tables 1 and 2.

Steering Stability

The test tires were assembled on wheels having a rim size of 19×9.0 J,mounted on a test vehicle (four-wheel drive having an enginedisplacement of 2000 cc), set to be inflated to an air pressure of 240kPa, and loaded with a load equivalent to two passengers. Under acondition of a speed of 110 km/h, sensory evaluations for steeringstability were made by three test drivers on a test course formed of apaved road. The evaluation results were each scored by a 5-point methodwith the results of Conventional Example 1 being assigned 3 points(reference), and an average value of the scores of the three testdrivers was indicated. Larger scores indicate superior steeringstability performance at high speeds.

High-Speed Durability

The test tires were mounted on wheels having a rim size of 19×9.0 J,inflated with oxygen to an internal pressure of 240 kPa, and held for 30days in a chamber maintained at a chamber temperature of 70° C. and ahumidity of 95%. The pre-treated test tires in this manner were mountedon a drum testing machine with a drum with a smooth steel surface and adiameter of 1707 mm, and the ambient temperature was controlled to 38±3°C., the speed was increased from 120 km/h in increments of 10 km/h every30 minutes, and the running distance until failure occurred in the tirewas measured. Evaluation results are expressed as measurement valueswith Conventional Example 1 being assigned the index value of 100.Larger index values indicate superior high-speed durability.

Presence of Separation

After the tests of high-speed durability described above were performed,each test tire was disassembled and the presence of separation at theturned up end portion of the carcass layer was confirmed. The evaluationresults were labeled “Yes” if separation occurred and “No” if noseparation occurred.

TABLE 1-1 Conventional Comparative Comparative Example 1 Example 1Example 2 Fineness after dip treatment 3900 3900 6400 dtex Intermediateelongation at 2.9 3.7 3.0 sidewall portion under % 1.0 cN/dtex loadRatio G/R 0.41 0.55 0.55 Twist coefficient K 2200 2200 2200 Steeringstability 3.0 2.5 3.5 High-speed durability Index 100 103 87 valuePresence of separation No No Yes

TABLE 1-2 Example Example Example Comparative 1 2 3 Example 3 Finenessafter dip treatment 6400 6400 6400 6400 dtex Intermediate elongation at3.3 3.7 4.2 4.6 sidewall portion under % 1.0 cN/dtex load Ratio G/R 0.550.55 0.55 0.55 Twist coefficient K 2200 2200 2200 2200 Steeringstability 3.3 3.3 3.2 2.7 High-speed durability Index 103 105 106 108value Presence of separation No No No No

TABLE 2-1 Comparative Example Example Example 4 4 5 Fineness after diptreatment dtex 6400 6400 6400 Intermediate elongation at sidewall 3.73.7 3.7 % portion under 1.0 cN/dtex load Ratio G/R 0.43 0.50 0.60 Twistcoefficient K 2200 2200 2200 Steering stability 3.4 3.3 3.2 IndexHigh-speed durability 85 102 105 value Presence of separation Yes No No

TABLE 2-2 Comparative Example Example Example 5 6 7 Fineness after diptreatment dtex 6400 6400 6400 Intermediate elongation at sidewall 3.73.7 3.7 % portion under 1.0 cN/dtex load Ratio G/R 0.64 0.55 0.55 Twistcoefficient K 2200 1800 2000 Steering stability 2.6 3.3 3.3 High-speeddurability Index 107 95 100 value Presence of separation No No No

As can be seen from Tables 1 and 2, in contrast to the referenceConventional Example 1, the tires of Examples 1 to 7 achieved a highlevel of both steering stability and high-speed durability. Furthermore,no separation occurred even after the tests of high-speed durability. Onthe other hand, in Comparative Example 1, the fineness of the carcasscord after dip treatment was small, and thus steering stabilitydecreased. In Comparative Example 2, the intermediate elongation of thecarcass cord at the sidewall portion under 1.0 cN/dtex load was small,and thus high-speed durability deteriorated, and separation occurred intires after high-speed durability testing. In Comparative Example 3, theintermediate elongation of the carcass cord at the sidewall portionunder 1.0 cN/dtex load was large, and thus the steering stabilitydecreased. In Comparative Example 4, the ratio G/R was small, and thushigh-speed durability deteriorated, and separation occurred in tiresafter high-speed durability testing. In Comparative Example 5, the ratioG/R was large, and thus steering stability decreased.

The invention claimed is:
 1. A pneumatic tire, comprising: a treadportion extending in a tire circumferential direction and having anannular shape; a pair of sidewall portions respectively disposed on bothsides of the tread portion; and a pair of bead portions each disposed onan inner side of the pair of sidewall portions in a tire radialdirection; the pneumatic tire comprising at least one carcass layermounted between the pair of bead portions; the carcass layer beingformed from a carcass cord formed of an organic fiber cord havingfilament bundles of organic fibers intertwined together, a fineness ofthe carcass cord after dip treatment being from 6000 dtex to 8000 dtex,an intermediate elongation of the carcass cord at the sidewall portionunder 1.0 cN/dtex load being from 3.3% to 4.2%, and the organic fibersforming the carcass cord being a uniform material; and the carcass layerbeing formed from a body portion extending from the tread portionthrough each sidewall portion to a corresponding bead portion and afolded back portion folded back at each bead portion and extendingtoward a corresponding sidewall portion side, and having a contactregion at the sidewall portion where the body portion and the foldedback portion are in contact, and a ratio G/R of an interlayer rubbergauge G between a carcass cord within the body portion and a carcasscord within the folded back portion at the contact region to a corddiameter R of the carcass cord being from 0.50 to 0.60; wherein a twistcoefficient K of the carcass cord after dip treatment represented byFormula (1) is from 2300 to 2500:K=T×D ^(1/2)  (1) where T is an upper twist count of cord (counts/10cm), and D is a total fineness of cord (dtex).
 2. The pneumatic tireaccording to claim 1, wherein the intermediate elongation of the carcasscord at the sidewall portion under 1.0 cN/dtex load is from 3.5% to4.0%.
 3. The pneumatic tire according to claim 1, wherein an organicfiber forming the carcass cord is a polyethylene terephthalate fiber. 4.The pneumatic tire according to claim 3, wherein the fineness of thecarcass cord after dip treatment is from 6000 dtex to 7000 dtex.
 5. Thepneumatic tire according to claim 1, wherein the fineness of the carcasscord after dip treatment is from 6000 dtex to 7000 dtex.
 6. Thepneumatic tire according to claim 5, wherein the intermediate elongationof the carcass cord at the sidewall portion under 1.0 cN/dtex load isfrom 3.5% to 4.0%.